The present invention is directed to compressed air filtration systems, and particularly to apparatus for removing dirt, dust, oil, rust, and moisture from a flowing gaseous medium such as compressed air. More particularly, the present invention is directed to a compressed air filtration apparatus having passive heat exchanger fins for warming compressed air traveling through the apparatus.
Filters are used to remove contaminants from compressed air lines. Untreated compressed air contains tiny particles of dirt, dust, oil, rust, and moisture. It is known to install an in-line filter in a compressed air supply line to clean and dry the compressed air to minimize disruption of pneumatically operated tools and machinery. Two-stage compressed air filtration systems which operate to remove harmful moisture, oil, and other contaminants from compressed air lines are also known. For example, two-stage filtration systems are disclosed in U.S. Pat. Nos. 4,874,408 to Overby; 4,600,416 to Mann; and Re. 32,989 to Mann.
Liquid water is one of the most corrosive elements that can be present in a compressed air supply system. Liquid water causes rust to form in the various pipes, valves, cylinders, air-operated tools or machinery, and other apparatus included in a compressed air supply system. Unwanted liquid condensation can occur whenever moisture-laden compressed air is cooled to a temperature below the dew point temperature. Various approaches have been developed to dehumidify a flow of compressed air. The moisture content of the compressed air after it exits from a moisture-removal filter varies by the type of unit that is used. Chemical, refrigerated, and mechanical dehumidifying systems are known.
Many chemical systems for drying compressed air have been developed to dehumidify compressed air and minimize corrosion problems in compressed air lines and pneumatic equipment. Desiccate dryers use a chemical or drying agent to absorb moisture present in a compressed air stream. For example, hygroscopic chemical-based compressed air desiccating systems are disclosed in U.S. Pat. Nos. 4,519,819 to Frantz; 4,468,239 to Frantz; 4,891,051 to Frantz; 4,108,617 to Frantz; and 5,002,593 to Ichishita et al. However, desiccant chemicals can be expensive and require periodic replacement or regeneration for continued use.
It is also known to use various power-consuming refrigeration units to cool and dehumidify compressed air. Such systems are expensive to use because they require external means such as electricity and/or water to operate. In addition, refrigeration units are expensive to maintain and service. In some cases, the supply of compressed air to an entire plant or module must be shut down during maintenance and service operations. Consequently, maintenance is often neglected leading to the presence of excess moisture and other contaminants in the compressed air supply system.
Another system for drying compressed air is based on purely passive mechanical elements. As compared to chemical-based systems, passive mechanical systems may be virtually maintenance-free, requiring at most only infrequent filter replacements. Such mechanical drying systems can use baffles, impingement plates, inverse flow, evaporation, or wire mesh coalescers to remove water from compressed air. For example, U.S. Pat. No. 4,668,256 to Billiet discloses the use of baffles and impingement plates; U.S. Pat. Nos. 4,822,387 and 4,957,516 to Daniels disclose an inverse flow filter assembly having internally disposed heat exchange cooling fins; and U.S. Reissue Pat. No. 32,989 to Mann discloses a two-stage wire mesh coalescer and evaporator cartridge. Other passive mechanical systems for drying compressed air are disclosed, for example, in U.S. Pat. Nos. 4,600,416; 4,874,408; 5,030,262; 4,385,913; and 2,512,785.
Although passive mechanical systems have many advantages, the moisture content of air passed through a mechanical air drying system is typically much higher than that of air passed through desiccating systems. For example, while desiccant-based drying systems may generate very dry air having a dew point as low as -40.degree. F., a conventional mechanical filter may produce dry air having a dew point normally ranging from 10.degree. to 35.degree. F. at normal atmospheric pressure. At higher pressures encountered with the use of compressed air, the dew point temperature correspondingly rises. For example, compressed air at 100 PSIG has a dew point of between about 60.degree. to about 90.degree. F.
A high dew point temperature increases the chance of water condensation from water vapor present in compressed air in an air line, pneumatic tool, or other air-operated machine. If ambient air temperature is below the dew point, compressed air leaving the mechanical drying system may spontaneously cool below the dew point during its passage through a downstream compressed air line or cool during expansion in the air tools, thereby causing condensation of damaging liquid water in the air line or tool and increasing opportunities for corrosion damage to air lines and air-operated tools. In practice, the possibility of such unwanted condensation has limited use of passive mechanical compressed air drying systems, maximum air line length after a compressed air filter station to relatively short distances (less than about 25 feet), or required multiple spaced-apart filters for longer distance air lines.
A passive compressed air filtration apparatus for drying compressed air to produce clean, dry, and warm compressed air without using expensive chemicals and electricity would be a welcome improvement. As long as the cleaned and dried compressed air discharged from such a filtration apparatus had a warm enough temperature (one that was above the prevailing dew point temperature), then the opportunity for any remaining water vapor present in the discharged compressed air to condense in the compressed air supply line, valves, or various pneumatically operated tools or machinery would be kept to a minimum.
According to the present invention, an apparatus is provided for filtering compressed air. The apparatus includes an air treatment unit having an inlet and outlet, a water vapor filter, and a sump. The water vapor filter provides means for expanding compressed air introduced into the air treatment unit to cool the compressed air. This promotes coalescence of water vapor in the compressed air that passes through the air treatment unit into droplets. The sump is an enclosure that provides means for receiving droplets produced by the water vapor filter and for conducting compressed air passing through the air treatment unit toward the outlet. By increasing the moisture-retaining capacity of compressed air, it is possible to minimize condensation in air lines and tools and the corrosion problems that are caused by such condensation.
The sump includes fin means for transferring heat from the surroundings to warm the cooled compressed air exiting the water vapor filter as it passes through the sump when the temperature of the surroundings is higher than that of cooled compressed air. The flow of compressed air is heated as it flows through the sump so that the moisture-retaining capacity of the compressed air is enhanced before it is discharged from the air treatment unit through the outlet. By increasing the moisture-retaining capacity of compressed air, it is possible to minimize condensation in air lines and tools and the corrosion problems that are caused by such condensation.
In preferred embodiments, the sump is provided by a hollow housing formed to include a chamber. A porous structure is positioned to occupy a lower portion of the chamber and configured to include honeycomb cells for receiving and storing water droplets produced by the water vapor filter. Compressed air exiting the water vapor filter flows above the porous structure through an upper portion of the chamber on its way toward the outlet of the air treatment unit.
The housing includes an upper shell coupled to a lower shell. Each shell is made of a thermally conductive material such as aluminum and includes a plurality of integrally formed, spaced-apart fins. These external fins function as heat-absorbing elements that extract heat from the atmosphere and transfer it into the chamber to heat the somewhat cool compressed air passing through the chamber.
The apparatus may also include a second-stage filter positioned to filter moisture and other contaminants from compressed air that has exited from the sump and is flowing toward the outlet of the air treatment unit. Any remaining moisture will be trapped in this second stage filter to ensure that only clean and dry compressed air is discharged from the air treatment unit.
In operation, unwanted moisture is removed by expanding the compressed air passing through the water vapor filter. This expansion cools the compressed air to a low temperature and promotes condensation of water vapor into water droplets. These water droplets can coalesce in the water vapor filter to form large droplets that fall under gravity into the underlying sump where the droplets are trapped in the porous honeycomb structure. At the same time, the cool and dry compressed air flows along a path above the porous structure through the sump and toward the second stage filter. Therefore, moisture removal is advantageously accomplished using a reliable and economical passive mechanical system without relying on any costly chemical-based desiccant drying systems or energy-consuming refrigeration systems.
When the ambient air temperature is higher than that of the cool and dry compressed air, the fins on the housing absorb heat from the atmosphere and transfer it into the sump to heat the cool compressed air to a preferred warmer temperature. Ideally, the compressed air is warmed to a temperature that is about equal to the temperature of the compressed air admitted into the air treatment unit through the inlet. Advantageously, such warming takes place using only heat extracted from the atmosphere around the housing without need for any electric heaters or the like. This warming results in discharge of a compressed air flow that is warm enough to hold and carry larger amounts of water vapor so that the possibility that condensation will occur and cause corrosion problems to develop in pipes and tools downstream of the air treatment unit is minimized. It is more likely that condensation will occur if the compressed air discharged from the air treatment unit is too cool because cool air is unable to retain a lot of moisture.
Essentially, the filtering apparatus in accordance with the present invention provides an adiabatic evaporation device that is designed to treat relatively large areas of a compressed air system. This device uses coalescing and adiabatic evaporation where no temperature change occurs in the inlet to outlet air temperature. Advantageously, by using adiabatic evaporation (humidification) and not cooling the air exiting the filtering apparatus, the distance or area of protection (from condensation by air temperature reduction) is extended over conventional filtering apparatus.
The extended distance or protection area between the filter and the point of use is beneficial as it increases the area that a single filter can service. By sizing and installing the air filter correctly, a single filter one size larger than two smaller filters can often be used. This in turn requires fewer filters and elements and lowers the yearly operating costs of maintaining clean dry air in a plant.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.